Lakes monitoring

Colorado has approximately 1,533 publicly owned lakes of greater than 10 surface acres. The total surface acreage of these lakes has been estimated at 164,029.

Significant publicly owned lakes are defined as those natural lakes, reservoirs or ponds where the public has access to recreational activities, such as fishing and swimming, or where the beneficial uses, such as water supply, affect the public.

Over the past 20 years, we’ve monitored 98 lakes and reservoirs across the state.

The lake and reservoir monitoring efforts provide data to evaluate the trophic status of Colorado lakes and reservoirs.

The data also are used to assess attainment of water quality standards.

At each lake, depth profiles of dissolved oxygen, pH, conductivity and temperature are collected at 1-meter intervals.

Water quality samples are taken from near the surface and near the bottom.

Samples are analyzed for a suite of chemical parameters, including nutrients, metals and inorganics.

In addition, the surface sample is analyzed for chlorophyll content as a measure of trophic status, and for the phytoplankton population to determine the algal species composition.

Trophic state is a classification of lakes based on the level of biological productivity (especially algae) and nutrient status.

Commonly used indicators of nutrient status and productivity include:

The amount of algae as measured by chlorophyll-a.

Water transparency as measured by Secchi disc depth.

In-lake epilimnetic total phosphorus concentration.

The trophic state is broadly defined as follows:

Oligotrophic: Lakes with few available nutrients and a low level of biological productivity characterized by clear water; often support cold-water fish species.

Mesotrophic: Lakes with moderate nutrient levels and biological productivity between oligotrophic and eutrophic; usually support warm-water fish species.

Eutrophic: Lakes with high nutrient levels and a high level of productivity; typically support exclusively warm-water fish species.

Hypereutrophic: Lakes in an advanced eutrophic state.

Trophic status is an index of water quality only to the extent that trophic condition limits the desired use of a lake (i.e., water supply or recreation).

Generally, the effects of lake eutrophication are considered negative, especially if the eutrophication is accelerated by human activities. Negative effects include:

Taste and odor problems for water supplies.

Reduction in water clarity, which is important for many recreational uses.

Reduction in the dissolved oxygen (DO) concentration in bottom waters to levels that are lethal to fish. Eutrophication often leads to increased fish production, but at the expense of desired species that inhabit cold, deep areas, such as trout.

As part of the lake assessments, we also consider data collected by other agencies. Routine monitoring of publicly owned reservoirs is being or has been performed by the U.S. Geological Survey, the Army Corps of Engineers, Denver Water and various other entities, including cities, regional councils of governments, and river basin associations.